Portable communication devices, such as mobile (cellular) telephones, portable computers, personal digital assistants (PDAs), Global Navigation Satellite System (GNSS) (e.g., including global positioning system (GPS)) receivers, and the like, are configured to communicate over wireless networks. Such portable communication devices may enable communication over multiple networks, and therefore include transmitters and receivers, as well as corresponding filters, power amplifiers, switches and other radio frequency (RF) components and devices, for example, connecting the receivers and transmitters to a common antenna, for sending and receiving RF signals over the various networks.
Many RF components and devices are nonlinear, and therefore generating intermodulation distortion (IMD), which is an indication of linearity. Examples of nonlinear components and devices include power amplifiers, low noise amplifiers (LNAs), acoustic filters and switches. Other types of RF components may have nonlinear characteristics, such substrates formed of nonlinear materials. IMD generally describes the ratio between the power of fundamental (operation) tones and second-order distortion (IMD2) products or third-order distortion (IMD3) products. That is, a linear RF component may produce a two-tone output signal, for example, where the frequencies of the two tones are the same as the frequencies of the two tones in the input signal. A nonlinear component outputs signals at frequencies in addition to the frequencies of the original two tones as a result of IMD products. These IMD products may cause interference in multi-channel communication equipment, where the IMD products (and/or higher harmonics of the IMD products) are generated at other operation frequencies in different frequency bands and/or in other networks. Examples of IMD2 products generated by a two-tone (first and second frequencies f1 and f2) input signal include f1+f2, f2−f1, 2f1 and 2f2. Examples of IMD3 products generated by a two-tone input signal include 2f1+f2, f1+2f2, 2f1−f2, 2f2−f1, 3f1 and 3f2. Notably, certain two-tone IMD3 products are typically close to the operation frequency/carrier frequency of the RF device, and therefore are very difficult or nearly impossible to filter out.
An additional requirement is the need for matching at higher harmonic frequencies, which requires a separate network (e.g., using short or open circuits), referred to as a harmonic matching network (or output network). The harmonic matching network ideally does not interact with the fundamental matching circuit, which provides impedance matching at the fundamental frequency of an RF voltage signal. With regard to power amplifiers in particular, the most important harmonic to remove at the transistor output typically is the second harmonic frequency. Conventionally, the second harmonic frequency is removed using a parallel resonant circuit, which was not difficult when the load is about 10K Ohms. However, assuming a load of 12.5 Ohms, for example, a transistor output of a power amplifier at 1.9 GHz provides values that at not realizable, as a practical matter. See, e.g., S. Cripps, “RF Power Amplifiers for Wireless Communications” (1999), p. 88, which is hereby incorporated by reference. This is particularly the case for mobile telephones, which generally include loads at about 4 Ohms, for example. Quarter wavelength transmission lines may also be used for harmonic matching. However the second harmonic of 3.8 GHz for application at 1.9 GHz (fundamental frequency), for example, requires a transmission line of about 19.7 mm, which is inconsistent with small package sizes.
FIG. 1 is a simplified schematic diagram of a conventional circuit including a nonlinear Class E power amplifier and harmonic matching network. Referring to FIG. 1, conventional amplifier circuit 100 includes a transistor 112, indicated by a switch, connected between voltage power supply (VDD) and ground via inductors 111 and 114, and connected in parallel with capacitor 113. The transistor 112 provides an amplified voltage signal to the output 118 for load 150, via the inductor 111 and series capacitor 116, in response to a variable drive signal (not shown) received by the gate or base of the transistor 112, depending on whether the transistor 112 is a field effect transistor (FET) or a bipolar junction transistor (BJT), for example. The amplified voltage signal at the output 118 has a fundamental frequency (f0).
As mentioned above, impedance matching at higher harmonic frequencies of the amplified voltage signal, in order to prevent interference caused by the higher harmonics, is attempted using only quarter wavelength transmission lines, indicated by representative transmission lines 121, 122 and 123, which provide the desired impedance at fundamental frequency. However, for frequency ranges up to about 5 GHz, and in particular for mobile phone applications, the quarter wavelength transmission lines 121 to 123 may be prohibitively long.